Until recent years, it was the general practice to backfill nearly all trenches with whatever material was dug out of the ground to make the trench. However, it has lately become the practice to lay plastic pipe containing relatively fragile products, including fiber optic lines, and other fragile materials in trenches and it has been found that if the initial backfill material is of a fine material (called "padding"), the fragile pipe or the like is better protected from damage from the courser backfill which inevitably follows. Where no suitable fine materials are readily available near at hand, it becomes advantageous to manufacture such materials on-site to avoid excessive transportation costs to bring such materials to the work site. Because much of this trenching work is done in rural areas and over relatively long distances, procurement of fine backfill material from off-site can present significant economic problems.
U.S. Pat. No. 4,633,602 issued to R. L. Leyh et al. for a "Method and Apparatus for Padding Pipe" describes a backfill machine which is operated over a berm alongside a trench containing newly laid pipe. It scoops up raw berm material in its path and feeds it by means of a longitudinally aligned conveyor belt to the upper end of a machine mounted screening system located near the after end of the backfill machine The screening system ("separator") is used to separate fine materials from course. A transverse conveyor belt, located beneath the separator, deposits the "fines" back into the trench, thereby "padding" the pipe or the like which has been laid in the trench. The course materials rejected by the separator are deposited on the ground behind the backfill machine adjacent the trench. The course materials are then deposited back into the trench over the fines by a bulldozer, grader or the like which follows along in the backfill machine's path for that purpose.
In U.S. Pat. No. 2,857,691, for a "Pipeline Ditch Filling and Pipe Padding Machine," M. Curran teaches a system intended for mounting on one side of a tractor or the like. It extends, boom-like, across the trench, perpendicular to the vehicle on which it is mounted and comprises a rotary scoop which guides the raw berm material onto a cage-like screening system located at the distal end of an auger/pipe conveyor tube. The fines are conveyed to the inner portion of the cylindrical screen and are transported by the conveyor tube to and through a hole in the bottom of the tube at or near the proximal (vehicle) end; said hole being located by the operator over the trench by steering the vehicle. The disclosure points out that the machine may be used to pad the bottom of the trench before the pipe is laid therein or it may be used for the first covering layer over the laid pipe, or for both functions. The course materials are deposited back to the berm just aft of the pickup point. An auxiliary feed is provided to mix concrete or the like with the fines to provide for stabilizing the mixture and preventing settling thereof.
U.S. Pat. No. 3,596,384, for an "Excavation Refill Packer," issued to R. E. Neujahr, describes a self-propelled machine which uses a belt mounted bucket conveyor which is fed by an off-center auger; that is, it is fed from one side to the longitudinal centerline of the moving machine where the bucket conveyor is mounted. The cup conveyor lifts the raw material from the trench berm to a position over the top of a screening system. A vibrating means for augmenting the screening process is suggested. The screened fines are deposited into the trench aft of the pickup point of the raw material. The chassis of the machine is arranged to be adjustable within a limited angular range with respect to the ground.
U.S. Pat. No. 4,664,791 to McLain et al. teaches yet another trench padding machine. It would appear to be a mobile screening mechanism which is fed from an auxiliary back hoe or the like (a second vehicle having earth moving capability). The screening system is supported over the trench from one side of the carrying vehicle; a tractor or the like. While the raw materials may be collected anywhere, they must be dumped into the hopper by another machine.
U.S. Pat. No. 4,116,014, issued to Satterwhite for an "Excavating and Pipeline Installation System," teaches a tractor/multiple trailer arrangement for excavating a trench, laying a pipeline therein and depositing the raw materials over the laid pipeline. The materials are deposited over the pipeline at the after end of the last trailer, each of which is equipped with a conveyor belt for transporting the material over that trailer's length. There is no teaching of a screening system for generating padding. Each trailer's conveyor belt begins beneath the after end of the conveyor belt of the previous trailer. By canting the belts upward at an after end thereof, this system acts to effectively move the materials from each forward trailer to the next succeeding one aft. Because the entire system straddles the trench which it digs, there is no need for transverse transportation of the materials. The last trailer merely dumps the material back into the trench which lies down the centerline of the combined vehicles.
None of the described apparatus delivers the fine material from a position with respect to the apparatus for producing the fine materials that would permit that same vehicle to also deliver courser materials into a trench over the fines in a single pass.